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Troubleshooting AWS NAT Instance Connectivity Issues: A Comprehensive Guide

Network Address Translation (NAT) instances in AWS sometimes encounter connectivity issues when forwarding traffic from private EC2 instances. Here's a detailed analysis of common problems and their solutions.

Key Configuration Points

Source/Destination Check

  • Disable source/destination checking on the NAT instance, as this is often the root cause of connectivity issues[3].

Network Configuration

  • Ensure the private subnet's route table directs 0.0.0.0/0 traffic to the NAT instance
  • The NAT instance must reside in a public subnet with proper routing to the Internet Gateway

Essential IPTables Configuration

# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Configure NAT rules
iptables -t nat -A POSTROUTING -o eth0 -s 10.0.0.0/16 -j MASQUERADE
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT
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Security Considerations

Security Group Settings

  • NAT instance security group must permit inbound traffic from private subnets
  • Allow all outbound traffic
  • Private instance security groups need outbound rules to NAT instance[1]

Connection Timeout Issues

The NAT gateway terminates idle connections after 350 seconds. To prevent this:

  • Implement TCP keepalive with values under 350 seconds
  • Maintain active traffic flow over the connection[1]

Advanced Troubleshooting

Network Connectivity Tests

ping 8.8.8.8
traceroute 8.8.8.8
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Consider using AWS NAT Gateway as an alternative for improved reliability and simplified management[1].

情報源
[1] Troubleshoot NAT gateways - Amazon Virtual Private Cloud https://docs.aws.amazon.com/vpc/latest/userguide/nat-gateway-troubleshooting.html
[2] NAT ゲートウェイのトラブルシューティング https://docs.aws.amazon.com/ja_jp/vpc/latest/userguide/nat-gateway-troubleshooting.html
[3] プライベートリソースが VPC の外側で通信できるようにする https://docs.aws.amazon.com/ja_jp/vpc/latest/userguide/work-with-nat-instances.html
[4] nat-instance(terraform-aws-nat-instance)を利用して発生した ... - Qiita https://qiita.com/fkdfkdfkd/items/09a15f32002dc44b6b18

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